Prerequisites

Note: Docker requires a 64-bit version of Ubuntu as well as a kernel version equal to or greater than 3.10. The default 64-bit Ubuntu 16.04 server meets these requirements.

All the commands in this tutorial should be run as a non-root user. If root access is required for the command, it will be preceded by sudo.

Step 1 — Installing Docker

The Docker installation package available in the official Ubuntu 16.04 repository may not be the latest version. To get the latest and greatest version, install Docker from the official Docker repository. This section shows you how to do just that.

First, add the GPG key for the official Docker repository to the system:

Installing Docker now gives you not just the Docker service (daemon) but also the docker command line utility, or the Docker client. We’ll explore how to use the docker command later in this tutorial.

Step 2 — Executing the Docker Command Without Sudo (Optional)

By default, running the docker command requires root privileges — that is, you have to prefix the command with sudo. It can also be run by a user in the docker group, which is automatically created during the installation of Docker. If you attempt to run the docker command without prefixing it with sudo or without being in the docker group, you’ll get an output like this:Output:

docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.

If you want to avoid typing sudo whenever you run the docker command, add your username to the docker group:

sudo usermod -aG docker ${USER}

To apply the new group membership, you can log out of the server and back in, or you can type the following:

su - ${USER}

You will be prompted to enter your user’s password to continue. Afterwards, you can confirm that your user is now added to the docker group by typing:

id -nG

Output:

sammy sudo docker

If you need to add a user to the docker group that you’re not logged in as, declare that username explicitly using:

sudo usermod -aG docker username

The rest of this article assumes you are running the docker command as a user in the docker user group. If you choose not to, please prepend the commands with sudo.

Step 3 — Using the Docker Command

With Docker installed and working, now’s the time to become familiar with the command line utility. Using docker consists of passing it a chain of options and commands followed by arguments. The syntax takes this form:

docker [option] [command] [arguments]

To view all available subcommands, type:

docker

As of Docker 1.11.1, the complete list of available subcommands includes:

Output:attach Attach to a running container
build Build an image from a Dockerfile
commit Create a new image from a container's changes
cp Copy files/folders between a container and the local filesystem
create Create a new container
diff Inspect changes on a container's filesystem
events Get real time events from the server
exec Run a command in a running container
export Export a container's filesystem as a tar archive
history Show the history of an image
images List images
import Import the contents from a tarball to create a filesystem image
info Display system-wide information
inspect Return low-level information on a container or image
kill Kill a running container
load Load an image from a tar archive or STDIN
login Log in to a Docker registry
logout Log out from a Docker registry
logs Fetch the logs of a container
network Manage Docker networks
pause Pause all processes within a container
port List port mappings or a specific mapping for the CONTAINER
ps List containers
pull Pull an image or a repository from a registry
push Push an image or a repository to a registry
rename Rename a container
restart Restart a container
rm Remove one or more containers
rmi Remove one or more images
run Run a command in a new container
save Save one or more images to a tar archive
search Search the Docker Hub for images
start Start one or more stopped containers
stats Display a live stream of container(s) resource usage statistics
stop Stop a running container
tag Tag an image into a repository
top Display the running processes of a container
unpause Unpause all processes within a container
update Update configuration of one or more containers
version Show the Docker version information
volume Manage Docker volumes
wait Block until a container stops, then print its exit code

To view the switches available to a specific command, type:

docker docker-subcommand --help

To view system-wide information about Docker, use:

docker info

Step 4 — Working with Docker Images

Docker containers are run from Docker images. By default, it pulls these images from Docker Hub, a Docker registry managed by Docker, the company behind the Docker project. Anybody can build and host their Docker images on Docker Hub, so most applications and Linux distributions you’ll need to run Docker containers have images that are hosted on Docker Hub.

To check whether you can access and download images from Docker Hub, type:

docker run hello-world

The output, which should include the following, should indicate that Docker in working correctly:

Output:

Hello from Docker.
This message shows that your installation appears to be working correctly.
...

You can search for images available on Docker Hub by using the docker command with the searchsubcommand. For example, to search for the Ubuntu image, type:

docker search ubuntu

The script will crawl Docker Hub and return a listing of all images whose name match the search string. In this case, the output will be similar to this:

In the OFFICIAL column, OK indicates an image built and supported by the company behind the project. Once you’ve identified the image that you would like to use, you can download it to your computer using the pull subcommand, like so:

docker pull ubuntu

After an image has been downloaded, you may then run a container using the downloaded image with the run subcommand. If an image has not been downloaded when docker is executed with the runsubcommand, the Docker client will first download the image, then run a container using it:

As you’ll see later in this tutorial, images that you use to run containers can be modified and used to generate new images, which may then be uploaded (pushed is the technical term) to Docker Hub or other Docker registries.

Step 5 — Running a Docker Container

The hello-world container you ran in the previous is an example of a container that runs and exits, after emitting a test message. Containers, however, can be much more useful than that, and they can be interactive. After all, they are similar to virtual machines, only more resource-friendly.

As an example, let’s run a container using the latest image of Ubuntu. The combination of the -i and -tswitches gives you interactive shell access into the container:

docker run -it ubuntu

Your command prompt should change to reflect the fact that you’re now working inside the container and should take this form:

Output:

root@d9b100f2f636:/#

Important: Note the container id in the command prompt. In the above example, it is d9b100f2f636.

Now you may run any command inside the container. For example, let’s update the package database inside the container. No need to prefix any command with sudo, because you’re operating inside the container with root privileges:

apt-get update

Then install any application in it. Let’s install NodeJS, for example.

apt-get install -y nodejs

Step 6 — Listing Docker Containers

After using Docker for a while, you’ll have many active (running) and inactive containers on your computer. To view the active ones, use:

The container-id can be found in the output from the docker ps command.

Step 7 — Committing Changes in a Container to a Docker Image

When you start up a Docker image, you can create, modify, and delete files just like you can with a virtual machine. The changes that you make will only apply to that container. You can start and stop it, but once you destroy it with the docker rm command, the changes will be lost for good.

This section shows you how to save the state of a container as a new Docker image.

After installing nodejs inside the Ubuntu container, you now have a container running off an image, but the container is different from the image you used to create it.

To save the state of the container as a new image, first exit from it:

exit

Then commit the changes to a new Docker image instance using the following command.

Note: If you run an image and made some modification and now want to save the modified image locally then follow the below steps:

use docker ps -a command to see the container listing and copy the container id.

now, use:

docker commit <container-id> <new-image-name>

It will save the modified modified image as the <new-image-name>. You can see the new modified image using the docker images command.

The -m switch is for the commit message that helps you and others know what changes you made, while -a is used to specify the author. The container ID is the one you noted earlier in the tutorial when you started the interactive docker session. Unless you created additional repositories on Docker Hub, the repository is usually your Docker Hub username:

docker commit -m "What did you do to the image" -a "Author Name" container-id repository new_image_name

Note: When you commit an image, the new image is saved locally, that is, on your computer. Later in this tutorial, you’ll learn how to push an image to a Docker registry like Docker Hub so that it may be assessed and used by you and others.

After that operation has completed, listing the Docker images now on your computer should show the new image, as well as the old one that it was derived from:

In the above example, ubuntu-nodejs is the new image, which was derived from the existing ubuntu image from Docker Hub. The size difference reflects the changes that were made. And in this example, the change was that NodeJS was installed. So next time you need to run a container using Ubuntu with NodeJS pre-installed, you can just use the new image. Images may also be built from what’s called a Dockerfile. But that’s a very involved process that’s well outside the scope of this article.

Step 8 — Pushing Docker Images to a Docker Repository

The next logical step after creating a new image from an existing image is to share it with a select few of your friends, the whole world on Docker Hub, or other Docker registry that you have access to. To push an image to Docker Hub or any other Docker registry, you must have an account there.

Conclusion

There’s a whole lot more to Docker than has been given in this article, but this should be enough to getting you started working with it on Ubuntu 16.04. Like most open source projects, Docker is built from a fast-developing codebase, so make a habit of visiting the project’s blog page for the latest information.

Please Note: The content of this page is collected from the https://www.digitalocean.com site for my personal reference in future use. Therefore, all the credit and copyrights goes to the mentioned site